1. Field of the Invention
The invention is in the field of Nuclear Magnetic Resonance (“NMR”) tools. More specifically, the invention pertains to new designs in NMR measuring devices in for obtaining azimuthal NMR images during measurement while drilling.
2. Description of the Related Art
Nuclear Magnetic Resonance has uses in many areas, including the fields of medicine, non-destructive testing, and in well logging in the oil exploration industry. In the well logging industry, NMR is used in determining properties such as porosity of the material, permeability, the bound liquid volume, the clay bound volume (CBW) and bulk volume irreducible (BVI), as well as formation type and oil content.
In many situations, it is desirable to obtain an azimuthal NMR measurement. This measurement may be used to determine formation characteristics such as porosity, bound fluid volume, T2, T1, and permeability. Being able to measure the azimuthal variation of these characteristics is useful for interpreting heterogeneous formations and performing geologically based steering in deviated or horizontal boreholes.
NMR logs in wireline instruments are typically made at much smaller logging speeds than other types of measurements. The stringent requirement placed on the magnitude of the static field gradient in the direction of motion is one reason. As the tool moves through the borehole, the formation volume in which the resonance condition is satisfied at the beginning of a pulse sequence must remain approximately the same through the entire sequence. If the longitudinal gradient is not small, then the sensitive volume will not remain constant. Another design requirement placed on NMR logging tools is the need to keep the RF antenna in place during the pulse sequences. If there is linear motion, previously excited nuclei move out of the sensitive volume and unexcited nuclei move in. Consequently, antennas with small apertures will have undesirable signal attenuation. U.S. Pat. No. 5,610,522 to Locatelli et al. discloses an open magnetic structure that could be used for making azimuthal NMR measurements but fails to include such a teaching and, due to the limited aperture, would have undesirable signal attenuation.
U.S. Pat. No. 5,977,768 to Sezginer et al. teaches the use of a segmented antenna for obtaining azimuthal information. The static magnetic field is produced by a pair of opposed magnets with magnetization parallel to the longitudinal axis of the tool. The region of examination is a toroidal zone around the borehole. By the use of segmented antennas, each antenna receives signals primarily from a quadrant. U.S. Pat. No. 6,255,817 to Poitzsch et al teaches a method for analysis of data from the Sezginer device. U.S. Pat. No. 6,326,784 to Ganesan et al. and EP 0981062A2 to Ganesan et al. disclose arrangements in which gradient coils are used to suppress spin-echo signals for portions of the region of examination. U.S. Pat. No. 6,373,248 to Poitzsch et al. discloses an azimuthally sensitive NMR tool in which the receiving RF antenna has a non-axisymmetric response.
As would be known to those versed in the art, the toroidal region defined by the opposed magnet configuration is relatively thin, as a result of which signal levels are low relative to a transverse-dipole magnet arrangement: restricting the region from which signals are obtained further lowers the signal level
It would be desirable to have a method and apparatus for making azimuthally sensitive NMR measurements that has an improved signal to noise ratio over the prior art methods identified above. The present invention satisfies this need.